(a) Field of the Invention
The present invention relates to a driving apparatus for a plasma display panel and a gray level expressing method thereof, and more particularly, to a driving apparatus for a plasma display panel and a gray level expressing method thereof that can reduce pseudo-contour.
(b) Description of the Related Art
Flat panel displays, such as a liquid crystal display (LCD), a field emission display (FED), a plasma display panel, or the like, have been developed recently. Among the flat panel displays, the plasma display panel has an advantage in that it has a wide visual range and that the brightness and light-emitting efficiency are high in comparison with other types of flat panel displays. The plasma display panel is in the spotlight as a display that can be substituted for the conventional cathode ray tube (CRT), especially in the large-sized displays of greater than forty inches.
The plasma display panel is a flat panel display that can display characters or images using plasma generated by gas discharge, on which hundreds of thousands or millions of pixels are arranged in a matrix format according to the size thereof. Such a plasma display panel is classified as a direct current type or an alternating current type according to the structure of discharging cells and the shape of the waveform of the driving voltage applied thereto.
The direct current type of plasma display panel has a shortcoming in that a current flows in a discharge space while the voltage is being applied as the electrodes are exposed to the outside while the discharge space is not insulated. Because of this a resistor for confining the current needs to be implemented. On the other hand, the alternating current type plasma display panel has an advantage in that the current is confined by capacitance formed naturally and the electrodes are protected by the impact from ions during the discharge by the dielectric layer covering the electrodes, so the lifetime is longer than that of the direct current type.
FIG. 1 is a partial perspective view of an alternating current type of plasma display panel. As shown in FIG. 1, scan electrodes 5 and sustain electrodes 5 covered by dielectric layer 2 and protection layer 3 are formed parallel in pairs on glass substrate 1. A plurality of address electrodes 8 covered by insulation layer 7 are formed on another glass substrate 6. Partitioning walls 9 are formed in parallel with address electrodes 8 on insulation layer 7 between address electrodes 8, and fluorescent substances 10 are formed on the surface of insulating layer 7 and both sides of partitioning walls 9. Glass substrates 1, 6 face each other with discharge spaces 11 between them so that scan electrodes 4 and sustain electrodes 5 are perpendicular to address electrodes 8. The discharge space near the intersection between address electrode 8 and scan electrode 4 and sustain electrode 5 that are coupled with each other forms discharge cell 12.
FIG. 2 shows an arrangement of the electrodes in the plasma display panel. As shown in FIG. 2, the electrodes in the plasma display panel are arranged in an m×n matrix form, and more particularly, address electrodes A1-Am are arranged in a column direction and n rows of the scan electrodes Y1-Yn and the sustain electrodes X1-Xn are arranged alternately in a row direction. Discharge cell 12 in FIG. 2 corresponds to discharge cell 12 in FIG. 1.
The driving period of such an alternating current type plasma display panel includes a reset time, an addressing time, and a sustain time according to the time flow of the change of the operation.
The reset time is the period to initialize the status of the respective cells in order to enhance the performance of the addressing operation of the cells, and the addressing time is the period to form a wall charge by applying the address voltage to the cells to be turned on (addressed cell) in order to select the cells to be turned on and not to be turned on in the panel. The sustain time is the discharge period for displaying the image actually on the addressed cells by applying sustain pulses.
As shown in FIG. 3, the plasma display panel realizes the gray level by dividing one frame (e.g., 1TV field) to a, plurality of subfields and then performing time-divisional control thereon. The respective subfields include the reset time, the addressing time, and the sustain time as described above. FIG. 3 shows the case in which one frame is divided into eight subfields in order to realize 256 gray levels. The respective subfields SF1-SF8 include a reset time (not shown), addressing time Ad1-Ad8, and a sustain time S1-S8, and in the sustain time S1-S8, the ratio of illuminating times 1T, 2T, 4T, . . . , and 128T is 1:2:4:8:16:32:64:128.
In such a situation, in order to realize the gray level of 3 for example, the sum of the discharging time is made to be 3T by discharging the discharge cells at subfield SF1 having illuminating time 1T and subfield SF2 having illuminating time 2T. The image of 256 gray levels can be realized by combining the subfields having different illuminating times as such.
However, while a moving picture is being displayed according to such a subfield method, pseudo-contour is generated due to the visual characteristics of a person. FIG. 4 shows an example of generated pseudo-contour. When an image in which gray level 127 and gray level 128 exist adjacently is moving rightward, such a status is expressed as FIG. 4 according to the subfield arrangement of FIG. 3. In such a situation, a person recognizes the gray levels in the direction of the dashed arrows shown in FIG. 4 according to the characteristics of the visual sense of the person that follows the movement of the image. Thus, a pseudo-contour such as the gray level 255 between the positions of gray levels 127 and 128 may occur.